1. Field of the Invention
The present invention relates to a harness assemblies. More specifically, the present invention relates to harness assemblies for rotating gimbals.
2. Description of the Related Art
In a gimbaled system, one or more gimbals are typically supported for movement relative to a stationary support member. In order to electrically connect components mounted on the gimbal with the stationary support member, a number of conductors are bundled together in what is conventionally referred to a harness assembly.
Conventional harness assemblies often employ a torsion-loop design. These designs exercise, stretch, bend or twist the conductors, resulting in the generation of both torsion and bending stress. The tighter the bend radius and the larger the dynamic range, the greater the stress on the conductors when the harness cycles through its range of motion. To ensure proper movement of the gimbal and harness, the packaging of the harness loop is made relatively loose.
However, the packaging for conventional torsion-loop designed harnesses usually does not allow for axial displacement. This can result in a moment that multiplies the restrictive force caused by the harness. This force translates against the motor driving the mechanism. As a result, the performance of conventional torsion-loop harnesses tends to degrade sharply with the addition of each conductor and shield added to the bundle.
Of major concern in any electrical circuit is Electro-Magnetic-Interference (EMI) which is commonly referred to as xe2x80x9cnoisexe2x80x9d. When least offensive, EMI still can serve to reduce the effectiveness of any electromechanical assembly. EMI can render such an assembly useless.
Another drawback of conventional harness systems resides in the fact that such systems function as miniature antennas by intercepting and introducing electro-magnetic waves into the assembly. Designers of conventional harness systems, in an attempt to reduce the effect of harness noise, first measure the input in magnitude and frequency and then incorporate filters into the design to compensate for the noise. This approach tends to be somewhat successful for stationary harnesses and antennas. However, the problem can not be solved so easily when the harness is dynamic as with cross-gimbal systems. In cross-gimbal harness assemblies, the harness not only picks up and amplifies existing EMI, but also actually creates EMI.
As is known in the art, when an electrical current passes through a wire, and that wire is moving through space, an electromagnetic field is created (Faraday""s motor principle). This phenomenon is referred to as capacitive interference.
Torsion-loop harnesses also tend to be very sensitive to G force inputs. When employed in a missile seeker assembly and the entire seeker is put under heavy G forces as occur during flight, the loop harness assembles are susceptible to xe2x80x9cflopping xe2x80x9d around and possibly interfering with one another or with the gimbal components. In addition, because torsion-loop harnesses require tight bend radii, the conductor strands and shielding may undergo some plastic deformation which, in turn, increases the force required to overcome or extend the harness. Plastic deformation also introduces work hardening of the conductor material in the harness. Work hardening occurs as the bend radius is extended and retracted in order to accommodate gimbal motion. In addition, over time the metal material in the bundle of conductors begins to xe2x80x9ccreepxe2x80x9d or cold flow at the location of the tight bend radius. Cold flow causes the loop harness assembly to develop a memory. This is undesirable in closed loop control systems. In effect, material work hardening and material creep adversely effect the performance of the loop harness over cycle time. When the performance of the loop is degraded, the performance of the gimbal will also be degraded.
Thus, there is a need in the art for a harness assembly that is easy to assemble, robust in nature, and exhibits performance that is exactly repeatable.
The need in the art is met by the unique, birdcage, torsion harness assembly of the present invention. The present invention provides a torsion harness assembly that avoids the undesirable characteristics associated with known torsion-loop harness assemblies. The birdcage harness assembly consists of a plurality of separate conductors, many twisted and shielded, that are capable of transmitting signals and power from a fixed position bulkhead support mechanism and nearby circuit cards to and across an outer gimbal to various gimbal components. The birdcage-shaped harness combines three separate harness designs extending to various components into one compressed bundle of continuous conductor strands that extend between the bulkhead and the outer gimbal. Preferably, the birdcage-shaped portion of the harness extends from the bulkhead along the axis of rotation of the gimbal into connection with block mounted on the gimbal from which a number of separate harness bundles extend to the gimbal supported components. By following the gimbal axis of rotation, the birdcage harness eliminates a moment arm multiplier and is thereby less sensitive to conductor/shield material on the bundle. In comparison, a conventional loop harness usually locates the bending stress away from the axis of rotation, creating a torque arm and increasing the force acting on the harness system.
When assembled, a first portion of the bundle of conductor strands are attached, preferably by gluing into a top potting block clamped to the rotating gimbal body. Individual conductor/shields exit the bottom of the clamp and are formed in equal lengths. While the actual number of conductors forming the bird cage portion of the harness assembly is considered a design choice, it has been found that up to approximately 35 conductors and/or shields can be bundled together in the present invention. Each of the conductors preferably extends substantially one (1) inch from the top potting block until being received in an opening formed in a lower potting block. The actual length of the various strands is considered a design choice, however, the lengths of the portions of the strands extending between the upper and lower potting blocks should be substantially the same. While the preferred embodiment of the invention employs continuous strands extending from the bulkhead to the gimbal components, it is considered within the scope of the invention to employ separate strands for each portion, which strands are connected to form a continuous electrical connection. A lower potting clamp mounted on the bulkhead is employed to frictionally compress the lower potting block into fixed attachment with the plurality of conductors forming the birdcage portion of the harness, thereby preventing the birdcage conductors from separating from the bulkhead. A plurality of separate bundles of conductors extend from the lower potting block to various components mounted on the bulkhead support mechanism, thereby forming electrical connections between the gimbal mounted components and bulkhead mounted components.
The lower potting block is mounted on the bulkhead or sensor support at a set distance from the upper potting block carefully determined such that the plurality of conductor strands extending from the gimbal to the bulkhead form a substantially bird cage configuration. In particular, the bundle of conductor strands are each bowed in a generally outwardly direction from the axis of the bundle which coincides with the axis of rotation of the gimbal member. The predetermined amount of xe2x80x9cbirdcagingxe2x80x9d or bowing of the various individual conductor strands may easily be controlled by controlling the axial separation of the lower and upper potting blocks.
At first inspection, the birdcage-shaped torsion harness of the present invention appears to move or navigate with the gimbal in pure torsion. While it is true that the bowed conductors do twist as the gimbal moves though an angle of rotation, it becomes clear that the individual conductor strands primarily move in bending. Because the individual strands are pre-compressed into their initial bowed or birdcage-shaped positions, movement of the gimbal has the effect of providing slack that is taken up in the strands as the gimbal rotates relative to the fixed position sensor. In effect, the conductor strands bend or stretch from their respective bowed configurations until they reach substantially linear shape. The conductor strands then bend back into their original bowed shapes as the gimbal rotates back to its original neutral position.
An advantage of the present invention resides in the effective reduction of the size of the harness package as compared to conventional loop harnesses. In particular, loop harnesses extending between the support and gimbal are required to be substantially longer than the preferred approximate one (1) inch length of the birdcage conductors. The loop conductors must be longer because they require a relatively large xe2x80x9cswingxe2x80x9d or range of motion per angle of rotation of the gimbal. Besides reducing the package size of the harness assembly, the birdcage design eliminates the high stress normally inherent in harness assemblies facing large angles of rotation of the gimbal. In addition, the birdcage harness is not G sensitive, and dramatically reduces spring torque and friction input to gimbaled system components caused by conventional loop harness assemblies.